Correctly return const references from min, max, identity, minimum, & maximum to agree with STL semantics.
Correctly return const references from min, max, identity, minimum, & maximum to agree with STL semantics.

--HG--
extra : convert_revision : svn%3A83215879-3e5a-4751-8c9d-778f44bb06a5/trunk%40315

sdalton1/thrust,sdalton1/thrust,thrust/thrust,marksantos/thrust,xiongzhanblake/thrust,jaredhoberock/thrust,andrewcorrigan/thrust-multi-permutation-iterator,GrimDerp/thrust,Ricardo666666/thrust,egaburov/thrust,thrust/thrust,zhenglaizhang/thrust,zhenglaizhang/thrust,sarvex/thrust,jaredhoberock/thrust,zeryx/thrust,Ricardo666666/thrust,dachziegel/thrust,mohamed-ali/thrust,GrimDerp/thrust,GrimDerp/thrust,marksantos/thrust,marksantos/thrust,dachziegel/thrust,xiongzhanblake/thrust_src,thvasilo/thrust,thrust/thrust,andrewcorrigan/thrust-multi-permutation-iterator,Ricardo666666/thrust,zhenglaizhang/thrust,raygit/thrust,zeryx/thrust,thvasilo/thrust,mohamed-ali/thrust,egaburov/thrust,xiongzhanblake/thrust,arnabgho/thrust,xiongzhanblake/thrust,egaburov/thrust,thvasilo/thrust,jaredhoberock/thrust,raygit/thrust,thrust/thrust,jaredhoberock/thrust,dachziegel/thrust,xiongzhanblake/thrust_src,mohamed-ali/thrust,arnabgho/thrust,sdalton1/thrust,raygit/thrust,zeryx/thrust,sarvex/thrust,sarvex/thrust,jaredhoberock/thrust,xiongzhanblake/thrust_src,andrewcorrigan/thrust-multi-permutation-iterator,arnabgho/thrust,thrust/thrust